As shown in FIG. 4, a shadow mask type color picture tube includes a panel 1, a shadow mask 2, a funnel 3, a deflecting yoke 4, a neck 5, an electron gun 6 and a screen 8. The shadow mask 2 in FIG. 4 is provided for allowing electron beams 7 to land properly on the screen 8 by sorting the electron beams 7 emitted from the electron gun 6.
The shadow mask 2 is made up of a body 9 of a shadow mask and a mask frame 10 as shown in FIG. 5. The body 9 and the mask frame 10 are assembled by welding a skirt portion 11 formed around the body and the inner surface of the mask frame 10 at the center positions of the long sides and short sides and at corner portions of the shadow mask.
Conventionally, the length L of the skirt portion 11 of the body 9 is almost the same over the whole skirt portion, i.e. in each center portion of the sides and in each corner portion, or in the case where a "boss" for reinforcing the mask frame 10 is provided in the inner part of each corner portion of the mask frame 10, in order to avoid that the length of the skirt in each corner portion is shorter than the length of the skirt in each center portion of the sides. In welding the body 9 and the mask frame 10, the deformation of the skirt portion 11 relates greatly to the deformation of the curved surface shape if the welding points are set at the upper part of the skirt portion 11 (the side near the curved surface of the body 9). On the other hand, there might be a case where the position of the welding points cannot be secured due to an assembly error or the like in the case where the welding points are set at the lower end part of the skirt portion 11. Thus, the appropriate position of welding points is usually about 2 mm above the lower end of the skirt portion 11.
Cut parts 12 are provided in the vicinity of the welding points of the skirt portion 11 and the mask frame 10, i.e. in the vicinity of each corner portion of the shadow mask 2 and each center portion of the long and short sides. The cut parts 12 are provided in order to prevent excess thickness in forming the skirt portion 11 by press-molding a sheet metal and the deformation of the curved surface of the mask, which is produced by a temperature rise of the body 9 caused by the collision of an electron beam while a color picture tube is working.
In such a shadow mask 2, the skirt portion and the mask frame 10 are welded after setting the skirt portion 11 into the mask frame 10 by pushing the skirt portion 11 into the mask frame 10 in order to maintain the curved surface of the body 9. Consequently, stress distortion tends to be left at the fixing points of the skirt portion 11 and the mask frame 10 and at the welding points pressed mutually by a welding machine.
When manufacturing a shadow mask type color picture tube, the shadow mask 2 is mounted to the inner side of the panel 1 that has been provided with the screen 8, and a frit sealing process, an exhaust process and the like are conducted for the panel 1 and the funnel 3. Therefore, the above-mentioned distortion remaining in the welding parts in the shadow mask 2 and in their circumference are released by heating or the like during such processes (a frit sealing process, an exhaust process and the like), resulting in deformation of the curved surface of the body 9. This deformation of the curved surface of the body 9 causes deterioration of the color purity in a color cathode-ray tube.
The materials used for the body 9 of a shadow mask include an aluminum-killed steel (hereafter referred to as an AK material) having a thickness of about 0.1-0.3 mm, an invar material (a low expansion material) that has been developed according to the recent trend toward large size and high intensity of a picture tube or the like. The mask frame 10 is generally formed using a soft steel. An examination as to the deformation of the body 9 during manufacturing processes was carried out for the 2 kinds of shadow masks (made of an AK material and made of an invar material). As a result, it was confirmed that the deformation in the shadow mask made of an invar material was greater than that in the shadow mask made of an AK material.
It can be considered that the difference in the deformation of the body 9 is caused by the difference in the thermal expansion in the body 9 and in the mask frame 10. A shadow mask made of an AK material (hereafter also referred to as "an AK mask", has almost the same coefficient of thermal expansion as the mask frame 10. However, the coefficient of thermal expansion of a shadow mask made of an invar material (hereafter also referred to as "an invar mask") is smaller than that of the mask frame 10. Consequently, it can be considered that the body 9 tends to be affected by the difference in the thermal expansion at welding parts.
Conventionally, a dummy baking treatment (for example, at least at 450.degree. C. for 40-50 minutes) that eliminates the deformation caused by the distortion beforehand by heating the shadow mask 2 has been conducted during manufacturing processes for the purpose of the prior elimination of various thermal deformation mentioned above created in the body 9 or the like by sealing, exhaust heating and the like of the panel 1 and the funnel 3.
In a shadow mask according to conventional techniques, in the case of assembling a color picture tube without conducting such a dummy baking treatment, a shift is created between a phosphor hole and an electron beam in the color picture tube.
FIG. 6 is a front view of a 36 cm (15 inch type) invar mask tube seen from the panel side as an example of a color picture tube according to the present invention. A screen has been formed on the inner surface of a panel 1. As shown in FIG. 6, phosphor holes having a unit made of red (R), green (G) and blue (B) are arranged regularly in the screen. Here attention should be paid to a phosphor hole unit 17 in the area near corner portions and a phosphor hole unit 18 in the area near center portions of the short sides. In FIG. 6, the phosphor hole units 17 and 18 are shown in an exaggerated manner to express the concept.
FIG. 7 shows a shift created between the phosphor hole and an electron beam. FIG. 7(a) shows an enlarged view of the phosphor hole unit 17 arranged in the area near corner portions in FIG. 6, and FIG. 7(b) shows an enlarged view of the phosphor hole unit 18 arranged in the area near center portions of the short sides in FIG. 6. In the area 17 near the corner portions of a color picture tube, an electron beam 20 shoots the outer side of a phosphor hole 19 as shown in FIG. 7(a), and in the area 18 near the center portions of the short sides of the color picture tube, the electron beam 20 shoots the inner side of the phosphor hole 19 as shown in FIG. 7(b), resulting in a shift between the electron beam 20 and the phosphor hole 19. It was confirmed that the total (d1+d2) of the shifted distance at landing in the area near the corner portions and in the area near the center portions of the short sides was 40 .mu.m as shown in FIG. 2 (explained below in detail).
FIG. 8 shows the change of the curved surface of a mask before and after conducting a heat treatment in the area near corner portions and in the area near center portions of short sides in a color picture tube. That is, FIG. 8(a) schematically illustrates the state of the change in curvature of the shadow mask surface, taken along the line I--I in FIG. 6, seen from the direction of the arrow. The alternate long and short dash line 22' and an unbroken line 22 show the reference curved surface before conducting a heat treatment for the mask and the curved surface after conducting a heat treatment for the mask respectively. FIG. 8(b) schematically illustrates the state of the change in curvature of the shadow mask surface, taken along the line II--II in FIG. 6, seen from the direction of the arrow. The alternate long and short dash line 23' and an unbroken line 23 show the reference curved surface before conducting a heat treatment for the mask and the curved surface after conducting a heat treatment for the mask respectively. As shown in FIG. 8, the curved surface 22 in the area near the center portions of the short sides shifts in the direction approaching to the phosphor and the curved surface 23 in the area near the corner portions shifts in the direction away from the phosphor compared to the reference curved surfaces 22' and 23' respectively. It was confirmed that the total (d3+d4) of the shift d3 in the curved surface 22 in the area near the center portions of the short sides and the shift d4 in the curved surface 23 in the area near the corner portions was 40 .mu.m.
It was found from the measurement results of FIG's. 7 and 8 that the change of the curved surface and the properties of an assembled color picture tube corresponded. In the further examination in detail, it was confirmed that there was a correlation between the total (d3+d4 in FIG. 8) of the shift in the curved surface and the total (d1+d2 in FIG. 7) of the shift (between an electron beam and a hole) at landing in the area near the center portions of the short sides and in the area near the corner portions as shown in FIG. 9.
In order to obtain a precise color picture, it is preferable that the total (d1+d2 in FIG. 7) of the shift at landing is restrained to 15 .mu.m or smaller, and it can be found from FIG. 9 that in order to restrain the total to 15 .mu.m or smaller the total (d3+d4) of the shift of the curved surface needs to be not greater than 15 .mu.m.
In a shadow mask according to conventional techniques as mentioned above, as can be seen from the test (research) results, even if a body of a shadow mask is made of an AK material that deforms thermally in a relatively low level (not to mention in the case of using a body of a shadow mask made of an invar material that deforms greatly especially during a thermal process), the body deforms thermally through a frit sealing process, an exhaust heating process and the like for a panel and a funnel. Consequently, in order to obtain a shadow mask having better precision, it is necessary to assemble the shadow mask after eliminating the residual distortion by making the shadow mask deform sufficiently beforehand through a heat treatment. Therefore, a dummy baking treatment has been indispensable.
It is necessary that this dummy baking is conducted at not lower than 450.degree. as a maximum temperature and for not shorter than 40-50 minutes as a holding time as mentioned above, since the heating value provided to a shadow mask by the dummy baking must be greater than that provided by a heating process conducted later. It takes about 3 hours for the whole dummy baking process.
In manufacturing a color picture tube, it has been a big problem on production efficiency and production cost that the processes requiring enormous time and energy are necessary.